Electrohydraulic systems are widely deployed in earthmoving machines, and other applications requiring high mechanical power. Typically, in such systems, a pump supplies hydraulic fluid under pressure to a cylinder or other actuated device. Both the pump and cylinder operate under electronic control of the applied hydraulic fluids. In order to assure proper operation, the hydraulic fluid should be output at constant pressure.
So-called “variable displacement pumps” have been developed that can selectively increase or decrease fluid pressure so that a uniform output pressure can be maintained. The manufacturing tolerances and assembly matching requirements for such pumps, however, are typically high in order to maintain stable and precise control of the output fluid pressure.
Hydraulic cylinders are often controlled with linearized control schemes, as described, for example, in U.S. Pat. No. 5,666,806, whereby hydraulic cylinder movement and position are correlated to pump output pressure. In particular, a table is often created by selecting data related to specific pump pressure values and hydraulic cylinder parameters, and, based on such data, extrapolating further cylinder parameters with linear control algorithms. The hydraulic cylinder is thus controlled by determining the pump pressure, and then identifying corresponding hydraulic cylinder locations and velocities.
Linear control schemes, however, often require precision equipment that must be frequently calibrated, and may not perform well under extreme working conditions. Also, different linear control algorithms, as well as control elements (e.g. servos), may differ from one machine to the next, and one element to the next. Thus, an algorithm that may be suitable for one machine with particular control elements may not be suitable for another. Further, multiple high resolution sensors may be required in order to accurately detect pressures in various lines of the hydraulic system. Such sensors, however, substantially increase system cost.
In addition, linearized control schemes require accurate models of system behavior. A hydraulic system, however, cannot typically be modeled exactly, due to among other things, unpredictable system disturbances, environmental changes, and measurement noise. Thus, circumstances may arise in which linearized control schemes cannot precisely predict hydraulic cylinder movement, resulting in faulty operation.
The present disclosure is directed to overcome one or more of the shortcomings in the prior art.